Arboviral Disease -- United States, 1994

Arboviruses are mosquitoborne and tickborne agents that
persist in nature in complex cycles involving birds and mammals,
including humans. Characteristics of arboviral infection include
fever, headache, encephalitis, and sometimes death. In 1994, health
departments in 20 states reported 100 presumptive or confirmed
human cases of arboviral disease * to CDC. Of these, 76 were
California (CAL) serogroup encephalitis; 20, St. Louis encephalitis
(SLE); two, western equine encephalomyelitis (WEE); one, eastern
equine encephalomyelitis (EEE); and one, Powassan encephalitis
(POW). This report summarizes information about arboviral disease
in the United States during 1994.
Powassan Encephalitis

POW was serologically confirmed in a 49-year-old female
resident of Massachusetts who had onset of illness May 24. She
reported removing an engorged tick from her abdomen approximately
2 weeks before onset of symptoms. She was admitted to the hospital
on May 25 with a diagnosis of meningoencephalitis, which progressed
during the following 72 hours to encephalitis involving the brain
stem and basal ganglia. During hospitalization, the patient was
comatose for 3 days and required mechanical ventilation. On June
16, she was discharged to a rehabilitation center and, on July 25,
was transferred to a resident health-care facility. On examination
in August 1995, she had residual weakness in her right leg
requiring a brace. The patient's prolonged convalescence is
consistent with that reported for POW encephalitis.
California Serogroup Encephalitis

During 1994, a total of 76 human CAL serogroup encephalitis
cases were reported from 13 states: West Virginia (32 cases), Ohio
(14), Wisconsin (seven), Illinois (six), Minnesota (four), Indiana
and North Carolina (three each), Alabama (two), and Iowa, Kentucky,
Michigan, Rhode Island, and Virginia (one each). Patients ranged in
age from 6 months to 26 years (mean: 7 years). A total of 57 cases
(75%) occurred among males. Onsets of illness occurred in May (one
case), June (one), July (12), August (35), September (22), and
October (five).
St. Louis Encephalitis

During 1994, a total of 20 human cases of SLE were reported
from five states. Sixteen cases were reported in Louisiana; most
(14) occurred in urban New Orleans (Orleans and Jefferson
parishes). Three cases (in 44- and 60-year-old men and a
63-year-old woman) were fatal. Patients ranged in age from 12 to 78
years (mean: 46 years). Of the 16 cases, nine (56%) occurred among
males. SLE cases also were reported in residents of Riverside
County, California; Charlotte County, Florida; Forrest County,
Mississippi; and Harris County, Texas (one each). For the 20 total
cases, onsets of illness occurred in July (one case), August
(nine), September (nine), and October (one).
Western and Eastern Equine Encephalomyelitis

During 1994, two human cases of WEE were reported from Goshen
County in southeastern Wyoming; the cases occurred in a 40-year-old
woman and a 42-year-old man. One human case of EEE in a 67-year-old
man was reported from Iberville Parish, Louisiana.
Western and Eastern Equine Encephalomyelitis in Animals

Surveillance for arboviral disease includes cases in
susceptible animals because, during previous outbreaks, animal
cases preceded human cases by 2-3 weeks. During 1994, a total of
five WEE cases among horses were reported from three states: Idaho
(two cases), Wyoming (two), and Texas (one). WEE was isolated from
emus in Boulder County, Colorado (one), and Lancaster County,
Nebraska (one), and from a symptomatic pigeon in Stanislaus County,
California.

A total of 133 cases of EEE among horses were reported from 11
states: Florida (54 cases), South Carolina (20), North Carolina
(15), Michigan (12), Georgia (nine), Alabama and New Jersey (seven
each), Indiana and Louisiana (three each), Ohio (two), and Virginia
(one). In addition, EEE virus was isolated from other species in
five states. In Michigan, virus was isolated from two pheasant
flocks. In Florida, EEE virus was isolated from specimens of
viscera from a symptomatic duck and from 1-4-week-old piglets
during an epizootic in the Florida panhandle in which 50 of 90
piglets observed had objective central nervous system signs; the
number of deaths is unknown. In Georgia, EEE virus was recovered
from a litter of 3-week-old boxer puppies; three of five puppies in
the litter died. EEE cases in emus were reported from New Jersey
(10 cases), Florida (three), Georgia (two), and North Carolina
(one).

Editorial Note

Editorial Note: The findings in this report indicate that CAL
serogroup encephalitis remains the most frequently reported
arbovirus infection in the United States. Although the number of
CAL serogroup encephalitis cases has remained relatively constant
since the 1970s and was reported primarily from the Midwest, the
number of cases reported from the South has increased. For example,
in 1994, Alabama for the first time reported CAL serogroup
encephalitis cases, and Kentucky and Virginia -- which previously
had
reported a total of only six cases since 1964 -- each reported one
in
1994.

In general, SLE occurs as periodic focal outbreaks followed by
years of sporadic cases. In 1994, a small focal outbreak of SLE
occurred in urban New Orleans. Evaluation of case-patients by date
of illness onset and location suggests that the earliest cases
occurred among persons living within or in proximity to urban
public housing projects. Subsequent cases followed a pattern of
radial spread from the central urban area, although the small
number of cases preclude a definitive analysis. An investigation by
New Orleans Mosquito Control Board personnel found large
populations of immature and adult Culex pipiens quinquefasciatus
mosquitoes under housing units. Leaking sewer lines located in the
crawl space beneath these housing units provided an extensive and
ideal habitat for the SLE virus vector mosquito.

The POW case in Massachusetts in 1994 was the first reported
from that state. Previously, the most recent POW case in the United
States occurred in New York in 1978. POW virus is a tickborne
flavivirus most closely related to Russian spring summer and
Central European encephalitis viruses. Although understanding of
the epidemiology of POW virus in the United States is limited, the
virus appears to be widely distributed. In North America, Ixodes
cookei has been implicated as the principal tick vector, and virus
has been recovered from several rodent and carnivore species,
including the red squirrel, woodchucks, striped and spotted skunks,
foxes, short- and long-tailed weasels, and the white-footed deer
mouse. **

Human infections with POW virus occur infrequently, with
seroprevalence rates of 0.5%-4.0% in areas where the virus is
endemic (1). During 1958-1981, a total of 19 confirmed POW cases
among humans were reported in North America, primarily from the
northeastern United States and eastern Canada. Since 1981, five
additional confirmed cases have been reported from Canada: Quebec
(two, one fatal) (H. Artsob, Quebec Laboratory Center for Disease
Control, personal communication, 1995); New Brunswick (one) (2);
Ontario (one); and Nova Scotia (one) (M. Mahdy, Ontario Ministry of
Health Laboratory Services, personal communication, 1995). Based on
evaluation of the 24 total POW cases that occurred in North America
during 1958-1994, risk for infection may be highest in wooded areas
where potential contact with infected rodent or carnivore hosts or
tick vectors is greatest. Of the 24 cases, 21 occurred in persons
aged less than 20 years. Four of the acute infections were fatal,
and two patients died 1 and 3 years after onset as a result of
sequelae reported to be directly related to the disease.

Health-care providers should consider arboviruses in the
differential diagnosis of aseptic meningitis and encephalitis cases
during the summer months. Early identification of arboviral cases
is important to implement risk-reduction strategies (i.e., use of
vector-control practices, repellents, and changes in human activity
patterns). Serum (acute and convalescent) and cerebrospinal fluid
samples should be obtained for serologic testing, and cases should
be promptly reported to state health departments. New rapid
diagnostic techniques, including detection of immunoglobulin M
antibody in acute serum or cerebrospinal fluids, have facilitated
confirmation of arbovirus infections.

* At CDC, a confirmed case is defined as febrile illness with mild
neurologic symptoms, aseptic meningitis, or encephalitis with onset
during a period when arbovirus transmission is likely to occur,
plus at least one of the following criteria: 1) fourfold or greater
rise in serum antibody titer, 2) viral isolation from tissue,
blood, or cerebrospinal fluid; or 3) specific immunoglobulin M
(IgM) antibody in cerebrospinal fluid. A presumptive case is
defined as compatible illness, plus either a stable elevated
antibody titer to an arbovirus ( greater than or equal to 320 by
hemagglutination inhibition, greater than or equal to 128 by
complement fixation, greater than or equal to 256 by
immunofluorescent assay, or greater than or equal to 160 by
plaque-reduction neutralization test) or specific IgM antibody in
serum by enzyme immunoassay.

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